I answered based on “how much of a problem would it cause if this company went away” but I see now that the question was universally interpreted as “how much of your identity is based on your hatred for this company which doesn’t care about that?”
Facebook is totally disposable; almost no one’s life would be really different if it disappeared right now.
Amazon is a huge retailer, but there are alternatives to almost everything it does. (Though the disappearance of EC2 would be very disruptive)
Google Search could be directly replaced with Bing (given a couple of months for them to build out their server farms). The main losses would be GMail and YouTube, though again there are alternatives.
Apple is at either #4 or #2, depending on whether you use a Mac / iPhone or not.
Microsoft is the least dispensable by a country mile. Without Windows and its by-blows, almost all businesses and government departments would simply cease to function. Banks wouldn’t be able to dispense cash, and stores wouldn’t be able to take it. Perhaps not many safety-critical systems would be directly affected, but those whose life-support machines did fail would be the lucky ones, as they’d be spared the horror of surviving.
